Miniaturization, portability, and ever-increasing functionalities of consumer electronics continually drive printed circuit board manufacturing towards smaller and more densely packed boards. Increased circuit layer count, decreased core and laminate thicknesses, reduced copper line width and spacing, smaller diameter through-holes and micro-vias are some of the key attributes of high density interconnect (HDI) packages or multilayer PCB's.
Copper circuitry forming the circuit layout of the PCB is fabricated typically either by a subtractive process, or an additive process, or their combination. In the subtractive process, the desired circuit pattern is formed by etching downward from a thin copper foil laminated to a dielectric substrate where the copper foil is covered with a photoresist and a latent image of the desired circuit is formed in the resist after light exposure, the non-circuit area of the resist is washed away in a resist developer and the underlying copper is etched away by an etchant. In the additive process, the copper pattern is built upward from a bare dielectric substrate in the channels of a circuit pattern formed by photoresist. Further copper circuit layers are bonded together by partially-cured dielectric resin, often called “prepreg,” to form a multilayer assembly of alternating copper circuitry conductive layers and dielectric resin insulation layers. The assembly is then subjected to heat and pressure to cure the partially-cured resin. Through-holes are drilled and plated with copper to electrically connect all circuit layers and thus form a multilayer PCB. Processes for the fabrication of multilayer PCB's are well known in the art and described in numerous publications, for example, “Printed Circuits Handbook,” Sixth Edition, Edited by C. F. Coombs, Jr., McGraw-Hill Professional, 2007 and “Printed Circuit Board Materials Handbook,” Edited by M. W. Jawitz, McGraw-Hill, 1997. Regardless of the PCB structures and fabricating methods, it is essential to achieve good adhesion between the copper circuit layer and resin insulation layer. Circuit boards of insufficient adhesion cannot survive the high temperature of solder reflow and subsequent soldering, resulting in delamination of the board and electrical malfunctions.
The surface of the copper circuit as patterned is smooth; however, this smooth surface does not adhere well to the resin layer. It is theoretically known that increasing the contact area between the two dissimilar materials would increase the adhesion strength. To improve the bonding between the copper and the resin, most conventional approaches rely on creating a highly roughened copper surface to increase its surface area and introduce micro-ravines and ridges into the surface that act as mechanical bonding anchors to promote adhesion to the resin.
One of the most widely known and used approaches is the so-called “black oxide process” in which a black colored oxide layer having a rough surface is formed on top of the copper surface. The black oxide consists of needle-shaped dendritic crystals or whiskers of a mixture of cuprous oxide and cupric oxide of up to 5 microns in length. This large crystalline structure provides high surface area and mechanical anchoring effect and hence good bondability. U.S. Pat. Nos. 2,364,993, 2,460,896, and 2,460,898 to Meyer first describe the oxidation of a copper surface to a black oxide layer using an alkaline chlorite solution. Some exemplary disclosures of earlier efforts in applying this method to copper-resin bonding in PCB's include U.S. Pat. Nos. 2,955,974, 3,177,103, 3,198,672, 3,240,662, 3,374,129, and 3,481,777.
Although such needle-shaped oxide layer greatly increases the surface area and bondability, the dendritic crystals are fragile and easily damaged during the lamination process resulting in bonding failure within the oxide layer. Subsequent modifications to the oxide process have been focused on reducing the crystal size and therefore the thickness of the oxide layer to improve the mechanical stability by optimizing the reagent concentrations and other process parameters. Some notable improvements in this regard are represented by U.S. Pat. Nos. 4,409,037 and 4,844,981, where there are described formulations of an alkaline chlorite solution at specific concentration levels and hydroxide to chlorite ratios. U.S. Pat. No. 4,512,818 describes the addition of water soluble or dispersible polymer additives in an alkaline chlorite solution to cause a black oxide coating of reduced thickness and greater homogeneity. U.S. Pat. No. 4,702,793 describes a method of pre-treating the copper surface with sulfuroxy acid reducing agent to promote the rapid formation of a copper oxide. Other methods for forming black oxide layer include oxidation of the copper surface with hydrogen peroxide as described in U.S. Pat. No. 3,434,889, alkaline permanganate as described in U.S. Pat. No. 3,544,389, thermal oxidation as described in U.S. Pat. No. 3,677,828, and phosphoric acid—dichromate solution as described in U.S. Pat. No. 3,833,433.
One remaining problem associated with this oxide roughening approach is that copper oxides are soluble in acid; and serious delamination of the bonding interface occurs during later process steps which involve the use of acid. For example, as noted earlier through-holes are drilled through the multilayer board and plated with copper to provide interconnection of the circuit layers. Resin smear is often formed on the surface of the holes from drilling and must be removed by a desmear process which involves permanganate etch followed by acid neutralization. The acid can dissolve the copper oxide up to several millimeters inward from the surface of the hole, which is evidenced by the formation of a pink-ring around the through-hole owing to the pink color of the underlying copper. The formation of pink-rings corresponds to localized delamination and represents serious defects in the PCB's. These defects have become a significant bottleneck in the production of multilayer PCB's and extensive efforts have been extended in seeking further improvement in the oxide layer so that it is not susceptible to acid attack and such localized delamination.
Approaches to solving the pink-ring problem have largely involved post-treatment of the copper oxide. For example, U.S. Pat. No. 3,677,828 describes a method of first oxidizing the copper surface to form an oxide layer and then treating the oxide layer with phosphoric acid to form a glass like film of copper phosphate resulting in high bonding strength and acid resistance. U.S. Pat. No. 4,717,439 describes a process for improving the acid resistance of copper oxide by contacting the copper oxide with a solution containing an amphoteric element which forms an acidic oxide such as selenium dioxide. U.S. Pat. No. 4,775,444 describes a process of first forming copper oxide layer and then treating with chromic acid to stabilize and/or protect the copper oxide from dissolution in an acid.
A number of studies have shown that acid resistance is improved by first forming cupric oxide on the surface of the copper and subsequently reducing the cupric oxide to cuprous oxide or copper-rich surface. U.S. Pat. No. 4,642,161 describes a method of reducing the cupric oxide using a borane reducing agent represented by the general formula BH3NHRR′, wherein R and R′ are each selected from the group consisting of H, CH3 and CH2CH3. U.S. Pat. No. 5,006,200 describes reducing agents selected from the group consisting of diamine (N2H4), formaldehyde (HCHO), sodium thiosulfate (Na2S2O3) and sodium borohydride (NaBH4). U.S. Pat. Nos. 5,721,014, 5,750,087, 5,753,309, and WO 99/02452 describe reducing agents consisting of cyclic borane compounds, such as morpholine borane, pyridine borane, piperidine borane, etc. The most commonly practiced method of reducing cupric oxide to form cuprous oxide is by use of the reducing agent dimethylamine borane (DMAB). This approach has reduced the radius of the pink-ring to certain degree, but is still limited and has not solved the problem completely since cuprous oxide is not completely insoluble in an acid.
To overcome the problem mentioned above, U.S. Pat. Nos. 5,492,595 and 5,736,065 describe methods of reducing the copper oxide to copper while maintaining the needle-like structure of the oxide. However, such needle-like structure is mechanically unstable and suffers from crush-down during the lamination process. Alternative oxide coating processes have been developed subsequently. Some exemplary processes are described in U.S. Pat. Nos. 5,532,094, 6,946,027B2, 5,807,493, 6,746,621B2, 5,869,130, 6,554,948, and 5,800,859. These alternative processes produce highly roughed copper surface by combining the traditional oxidation process with a controlled etch that roughens the underlying copper surface while oxidizing it at the same time. In many cases, an organic layer is coated simultaneously to act as corrosion inhibitor or adhesion promoter. In U.S. Pat. No. 5,800,859, there is described a micro-roughening process using an etching agent comprising hydrogen peroxide, an inorganic acid, and a corrosion inhibitor such as triazole. U.S. Pat. Nos. 6,716,281B2, 6,946,027B2, 7,108,795 B2, 7,211,204 B2, and 7,351,353 B1 describe a similar approach for providing roughened copper surface using a composition comprising an oxidizer, a pH adjuster, a topography modifier, a uniformity enhancer, and an azole inhibitor. For the same purpose, U.S. Pat. Nos. 5,532,094, 5,700,389, 5,807,493, 5,885,476, 5,965,036, 6,426,020B1, and 6,746,621B2 describe a micro-etching composition consisting of an oxidizer like hydrogen peroxide, a cupric ion source, an organic acid, a halide ion source, and an azole type inhibitor. These approaches have increased the acid resistance; however, the interface bonding is achieved mainly by mechanical anchors and the adhesion strength diminishes rapidly as the surface roughness of the treated copper surface decreases.
As is readily seen, while numerous approaches have been developed for improving the adhesion between the copper surface and dielectric resin used in PCBs, the approaches have relied on creating a highly roughened surface to promote adhesion. It is universally thought in the prior art that the copper surface must be roughened to increase the surface area for bonding or adhering to the epoxy or dielectric resins. This approach however suffers from a serve limitation since the width and/or spacing of the copper lines is limited thus preventing further miniaturization of the circuitry of the PCB. The current trend toward higher density PCBs with finer line circuitry and increased layer counts has generated the need for higher bonding strength of copper to dielectric resins while retaining the smooth surface. Clearly, there is a present need for further advances and developments of PCBs, such as but not limited to a copper surface treatment process that increases the bonding strength and yet does not roughen the copper surface.